Technical Field
Embodiments of the subject matter disclosed herein generally relate to designature of marine seismic data or, more specifically, to performing azimuth and take-off angle dependent designature in shot domain.
Discussion of the Background
In geophysical prospecting, gas and oil reservoirs are sought by performing seismic surveys of sedimentary rock formations. Variations of seismic wave propagation velocity from one formation layer to another cause reflected, refracted and transmitted waves, some of which are detected by seismic receivers after traveling through the explored formations.
Traditionally, marine seismic data was acquired using one vessel towing a source and one or more streamers (i.e., cables carrying receivers). FIG. 1 is a bird's-eye view of such a conventional data acquisition system 100. The term “marine” is not limited to sea or ocean environments, but such systems may be used in any large bodies of water (e.g., freshwater lakes). Data acquisition system 100 includes a ship 120 towing plural streamers 140 that form a “spread.” The streamers may extend over kilometers behind ship 120 in the towing (in-line) direction, while cross-line distance (perpendicular to the in-line direction, in the horizontal plane) between adjacent streamers may be about 100 m. Receivers 160 (only a few are labeled) are disposed along streamers 140. Ship 120 also tows a source 180 (which may include plural individual sources such as air-guns) configured to generate a seismic (pressure) signal.
An in-line distance (i.e., parallel to towing direction) in the horizontal plane, between source 180 and the first receivers on streamers 140, may be a few hundred meters, while streamers' length may be up to 10 kilometers. Seismic signals generated by source 180 and propagating downward penetrate the seafloor and may be redirected when encountering layer interfaces (not shown in FIG. 1) inside the explored underground structure. Parts of the seismic signals emerging from the seafloor (i.e. after traveling through the explored formation) are detected by receivers 160.
Data acquired with system 100 in FIG. 1 is characterized by a narrow, limited azimuth angle range. An azimuth angle corresponding to a detected signal (e.g., φ for receiver 161 in FIG. 1) is defined in a horizontal plane, and is the angle between a line in the towing direction passing through the source's position (e.g., sail line S in FIG. 1) at the moment of the shot (i.e., when the signal has been generated), and a line connecting the source's position at the moment of the shot, and the receiver's position at the moment at which it detects a portion of the signal. If a seismic detector were positioned directly behind the source in the towing direction, the azimuth angle would have been 0°. The receivers at the front of the outermost streamers in the spread may have an azimuth angle of up to ±75°, but azimuth angles for the other receivers and on other streamers rapidly decrease to be mostly in the range of ±10°.
In order to achieve a stronger signal, the seismic source includes plural individual sources (e.g., air-guns) activated (e.g., fired) in a predetermined sequence. Each individual source has its own characteristics. For example, an air-gun is characterized by a minimum phasing and bubble energy. The signal generated by the source incorporates individual sources' contributions and has a shape (e.g., amplitude versus time) that varies with the distance from the source until, at a great enough distance from the source, the signal starts having a stable shape. This stable shape is known as the source signature. Once the signal's shape becomes stable, the signal's energy decreases inversely proportional to the distance from the source. The region where the signal's shape no longer changes with distance is known as the “far-field,” in contrast to the “near-field” region where the shape varies.
The signal penetrating the seafloor is thus characterized by the source signature, e.g., P(t). The signal recorded by a receiver, D(t), is considered to be a convolution of an impulse response, G(t) (which is determined by the explored formation's reflectivity), with attenuation E(t) and the source signature, P(t), plus some noise N(t):D(t)=[P(t)*G(t)*E(t)]+N(t).   (1)
Since both the impulse response and the attenuation are dependent on the explored formation's structure, a source signature's impact on the seismic data is removed during data processing (i.e., designature of the data). The source signature P(t) is measured or otherwise determined. Designature of the data is achieved by convolving the inverse of P(t) with D(t).
The source signature depends on the azimuth angle φ and the take-off angle α characterizing the data. This dependence is known as directivity. Conventionally, since, as discussed above, most of the data is characterized by azimuth angles in the range of ±10°, the same (i.e., a single) source signature has been conventionally used for designature, disregarding the source signature dependence of the azimuth angle.
As illustrated in FIG. 2, the take-off angle α for a source 210 and a receiver 220 is the angle with a vertical line made by a trajectory 230 of the signal as emitted by the source and partially detected by the receiver. The spatial extent of the source causes far-field directivity with take-off angle, while the source's asymmetry relative to the towing direction leads to the directivity with azimuth. Thus, the signature of the emitted signal varies both with azimuth and take-off angle.
To minimize source directivity, different source designs have been investigated, leading to a current design of a source shorter and narrower than in past decades, for which the source directivity effect decreases. With the current design, using a single source signature is almost correct for data with limited offset (i.e., distance from the source's position to the receiver's position) ranges and narrow-band spectra. However, because the acquisition technology extended azimuth coverage from narrow azimuth (NAZ) to wide azimuth (WAZ), and now to full azimuth (FAZ), ultra-long offset, and broadband, designature using a single source signature is no longer sufficient.
FIGS. 3 and 4 are graphical illustrations of source directivity for an azimuth angle of 0° and 90°, respectively. The source whose signature is illustrated in these graphs includes four arrays (i.e., substantially parallel towed cables) along which at seven substantially similar cross-lines positions are placed a total of 34 active air-guns with a total volume of 6,470 in3. In FIGS. 3 and 4, frequency increases along radii up to 90 Hz, and take-off angle varies around the semi-circular graph. Variations in intensity (i.e., amplitude), are seen in the patterns as differences in gray-scale. For azimuth 0° (in FIG. 3), the source directivity has a bigger variation in the spectrum domain when the take-off angle becomes larger than 50°, while for azimuth 90° (in FIG. 4), the spectrum begins varying from take-off angle 30°.
Hydrophones placed close (e.g., 1-2 m above each air-gun) have been used to estimate and monitor source signature shot-by-shot. Recently, (see U.S. Patent Application Publication No. 2014/0043936 by Poole et al., which is incorporated herewith in its entirety by reference) a method using these hydrophone measurements applies a shot-to-shot directional designature. The method performs a τ-p transform on the data in the common receiver domain and applies the corresponding directional designature filter on the p traces that represent the energy from different take-off angles. This method is affected by the sparseness of the data in the common receiver domain.
It is desirable to develop methods able to perform designature of marine seismic data that take into consideration the source signature's directivity and mitigate drawbacks of existing methods.